In recent years, the Federal Government has proposed to allow multiple tiers of users to share access to radio frequency spectrum in a controlled manner as opposed to unlicensed use, where all users are unprotected from interference from others, or licensed exclusive use, which only allows one licensed user access. A present proposal would authorize a set of channels within an available range of frequencies in a shared radio frequency spectrum model. In this shared radio frequency spectrum model, a set of channels may be at any frequency within the recently made available larger frequency band. In other words, for example, a license may be purchased for a 10 kHz channel in the frequency range of 20 MHz-22 MHz. Under a proposed licensing arrangement, any device would be allowed to operate within the 20-22 MHz range, but only on a temporarily assigned 10 kHz channel within the range. This new Government licensing proposal is like a general seating arrangement in a movie theater, where there are no assigned seats. Instead, the ticket holder is seated anywhere the usher directs as opposed to purchasing a designated specific seat ahead of time. A user in the proposed shared access radio frequency system would not have a statically assigned channel or set of channels at a specific frequency or a specific sub-block, but instead would have a potentially different channel assigned from time to time.
One proposal establishes a tiered priority access system for the Shared Access Systems (SAS). The first tier (Tier 1) is reserved for Government and military incumbent users who are the highest priority and the highest power-emitting users. The next tier (Tier 2) may be Priority Access/Commercial Wireless Network Providers (e.g. Verizon®, AT&T® and the like), who are proposed to have a mid-level priority and may have medium power emission levels, and the third tier (Tier 3) of users, known as General Authorized Access (GAA) users that have the lowest priority and lowest power emission levels.
The managing of the shared spectrum for the disparate devices is complicated because the different tiers of users may interfere with one another's use of a particular communication channel. Some proposals utilize a radio frequency (RF) propagation model to determine how to properly assign the channels, based upon parameters such as the channel frequency, the power levels and antenna gains, and the distances between the various users. By using propagation models to determine the possible interference, the manager can supposedly make a better assignment of the channels to avoid co-channel and adjacent channel interference because the users are too close or operating at too high a power level. The propagation modeling is based on the transmit and receive capabilities of the devices operating within the shared access system to make a determination of which particular communication channel should be assigned to respective devices in an area being serviced by an access point or base station device. However, accurate modeling of the RF propagation of the respective devices requires extensive, precise, and accurate information (such as, for example, three dimensional geometries of all landscape and structures surrounding the respective devices, dielectric constants for all bulk materials and conductivity figures for all surfaces, as well as other information). Also, fine grain motion of objects (e.g., time variation of the environment) due to moving people, moving vehicles, swaying of trees, etc., also all cause substantial time variation (e.g., signal fading). As a result, most practical RF propagation models are crude and approximate. Therefore, accurate prediction of path loss from a transmitting co-channel interferer to a candidate victim receiver at a known location is extremely difficult, and RF modeling techniques are only approximate and are often conservatively biased, meaning that in order to prevent interference, the prediction uses optimistic propagation distances (typically longer than actual) or path losses (typically more loss than actual) in order to avoid possible interference. Based on the conservative bias toward preventing interference, areas in which a candidate victim receiver may operate may be smaller than appropriate and have arbitrary exclusion boundaries that unnecessarily limit possible channel reuse. Often, the boundary contours are simple circles (having radii that represent a minimum ‘keep-away’ distance) that do not actually consider local conditions.
This conservatism leads to larger distance spacing between allowed co-channel users because, for example, due to the longer than actual propagation distances, thus resulting in longer distances for allowed frequency reuse, which actually results in less frequency reuse. Since frequency reuse is a metric for optimizing overall spectrum efficiency (e.g., how much total aggregate data can be handled by an air interface within a given amount of spectrum bandwidth), this conservatism leads to inefficient use of the shared radio frequency spectrum. In modern systems, this inefficiency is tolerated because the threat of interference is great, and interference can make the RF channels unusable if the victim receives too much interference. It is preferred to have a reliable RF channel for the transmission of voice and data signals, so excessive interference is preferably avoided. In order to minimize the amount of interference on the authorized channels, the modern systems typically err by being overly conservative regarding channel reuse distances.
These difficulties are further exacerbated in a multi-tiered shared radio frequency system because the various groups of users may deploy with differing air interfaces that use different transmitters and receivers, with differing power levels, bandwidths, waveforms, modulation methods, demodulation methods, and error protection schemes that allow operation at different resultant signal to noise ratios. These disparate system configurations further complicate use of a simple database or a simple propagation model in trying to assess the amount of interference a given device would receive from the surrounding disparately configured systems' interfering transmitters.
Other systems, such a conventional autonomous dynamic spectrum allocation (DSA), allow a local base transceiver system or access point itself to autonomously sample the radio frequency environment and determine which radio frequency channels to assign to itself. Autonomous DSA behavior does not provide information to a spectrum manager, so the autonomous local base transceiver systems or access points operate on a contention basis, and intelligent assignment (i.e., proactive management or coordination) of communication channels does not occur.
Hence a need exists for more accurate determinations of the interference between disparate systems in a shared access radio frequency spectrum so more accurate interference estimates may be made, and the intelligent assignment of communication channels may occur.